Add missing meta file in Zenject package

[MrCheez]

Pull request checklist

Please check if your PR fulfills the following requirements:

• Tests for the changes have been added (for bug fixes / features)
• Docs have been reviewed and added / updated if needed (for bug fixes / features)
• No compiler errors or warnings

Pull request type

Please check the type of change your PR introduces:

• Bugfix
• Feature
• Code style update (formatting, renaming)
• Refactoring (no functional changes, no api changes)
• Build related changes
• Documentation content changes
• Other (please describe):

Issue Number

Issue Number: #221

What is the current behavior?

• The Unity package version is missing a .meta file for the OptionalExtras/Async folder
• Adding Zenject via UPM results in a UPM exception:
  

What is the new behavior?

• The .meta file of the UnityProject/Assets/Plugins/Zenject/OptionalExtras/Async folder has been added to the repository
• Cloning the Zenject package via UPM no longer results in a missing meta file for immutable files exception

Does this introduce a breaking change?

• Yes
• No

Other information

On which Unity version has this been tested?

• 2020.4 LTS
• 2020.3
• 2020.2
• 2020.1
• 2019.4 LTS
• 2019.3
• 2019.2
• 2019.1
• 2018.4 LTS

Scripting backend:

• Mono
• IL2CPP

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[MrCheez]

Is there anything I can do to speed up the merge/review process @Mathijs-Bakker @nonumpa?

[nonumpa]

I have no idea. I approved just because I also need this fix.